In a wire-cut electrical discharge machine, machining is effected by applying voltage between a wire electrode and a workpiece, thereby causing electrical discharge between the wire electrode and the workpiece. An insulating machining fluid is supplied between the wire electrode and the workpiece, so that insulation between the wire electrode and the workpiece is recovered with a termination of every cycle of electrical discharge. After the insulation is recovered, voltage is applied again between the wire electrode and the workpiece, thereby causing an dielectric breakdown and resulting electrical discharge. If another electrical discharge is started without recovering the insulation between the wire electrode and the workpiece after the preceding electrical discharge, the electrical discharges occur in one and the same place (concentrated electrical discharge), thereby adversely affecting the accuracy and quality of the machined surface. If such concentrated electrical discharge has occurred, the machining will have to be suspended.
Conventionally, in order to check whether or not the insulation is recovered, a voltage from another power source, separate from the power source for electrical discharge machining, is applied between the wire electrode and the work piece during the off-time in which no voltage for machining is applied between the wire electrode and the workpiece, and the voltage between them are detected.
If a special power source separate from the power source for machining is used, as in the conventional method, in order to detect the recovery of the insulation between the wire electrode and the workpiece, it needs to provide such power source and its control circuit which render the wire-cut electrical discharge machine costly.